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Mike, ASE Certified Master Technician
Category: Nissan
Satisfied Customers: 417
Experience:  19yrs Automotive and Bus experience. Owner/Operator/Tech of Auto repair facility
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I've got a p0420 upstream code after replacing whole exhaust

Customer Question

I've got a p0420 upstream code after replacing whole exhaust system as well as catalytic converters and 02 sensors, 1 of the sensors I replaced must be bad, can this be easily targeted and what kind of cost?
Submitted: 1 year ago.
Category: Nissan
Expert:  Mike replied 1 year ago.

It has been my experince that that code is never resolved with oxygen sensors. I also have experienced multiple times that aftermarket catalytic converters are not strong enough to pass the Catalyst monitor, So they set the P0420. What yo most likely need is a factory cat. Or Try a different brand of aftermarket cat.

Customer: replied 1 year ago.
would it mean I need to replace both cats or the upstream only
Expert:  Mike replied 1 year ago.

What is the year, Make, Model and engine size of this vehicle?

Customer: replied 1 year ago.
Also, would a Walker Catalylidic converter be considered after market?
Expert:  Mike replied 1 year ago.

Anything not Made by the original manufacturer is aftermarket. Walker is aftermrket

Customer: replied 1 year ago.
Sorry mike it's a 2004 Nissan Maxima sl
Expert:  Mike replied 1 year ago.

The code you have is refering to the Precatalytic converter attached to the rear manifold by the firewall. Its about $700 from nissan.

Expert:  Mike replied 1 year ago.

You would normally want the precats to match. Or it may set another code for the bank 2 after you replace the bank1

Customer: replied 1 year ago.
Thank you much mike!...
How long do I have to communicate with you on this topic?
Expert:  Mike replied 1 year ago.

Im not exactly sure about that